Numerous applications exist which require the use of an illuminated sign for night-time visibility in situations where access to an electric mains supply is either unavailable or inconvenient to supply. Some such applications are in house signs, road signs, directional signs, road milestones, camping sites, emergency signs such as in aircraft or in underground car parks, display signs for advertising purposes or for providing other information. Such signs should ideally have uniform illumination over the whole of the area in which the sign's information is to be displayed, should be visible by day as well as by night, and should provide a level of illumination visible at some tens of meters at night with a minimal electrical power input.
In the prior art, many different types of such signs exist, and in many of them, the illumination source is generally internal to the sign structure, and is powered by a secondary cell or battery, which is maintained charged by solar power. One of the main problems to be solved in all such internally illuminated signs is the provision of uniform illumination without any regions of higher or lower intensity. Reflectors and/or diffusers are generally necessary in order to improve uniformity. Large areas arrays of fluorescent tubes are quite effective to this end, but are very energy consumptive.
A number of prior art, internally illuminated signs have been described using light emitting diodes (LED's), which are generally more energy saving than incandescent or fluorescent sources, because of their ability to localize their emission in the direction of desired illumination. Some examples of such prior art signs are to be found in U.S. Pat. Nos. 5,105,568, 5,539,623, 5,388,357, 5,729,925 and 4,952,023, and in International Publication No. WO 03/010738, all of which are hereby incorporated by reference, each in its entirety. In such signs, reflective surfaces are generally used within a light box to direct the illumination towards the sign panel. The reflective surfaces can be either on the back of the character panel, or on other internal reflective surfaces, or usually on both. Commercial products are available for facilitating these functions, such as those supplied by the 3M Company of St. Paul, Minn. 55144, which provides a range of products designed for enhancement of the performance of internally illuminated signs. One such product is called a blockout film, products 3635-20B and No. 3635-22B, and is designed for preparing the indices of such signs. These films have a black matte surface on one side, which is used as the outer side of the sign, and a white surface on the other side, used to reflect light internally from the back of the indices.
Many of the above-described prior art signs have a number of disadvantages. Firstly, they often use a source located at one end of a light box with internal reflective surfaces, and although this is a convenient arrangement for such signs, the design of such prior art signs does not generally provide sufficiently uniform illumination on the sign panel. Secondly, they generally use flat panels, and a source located at one or both ends of a light box may generate an illumination gradient down the length of such a flat panel sign. Thirdly, the manufacturing method of the panel with the display character, such as is described in WO 03/010738, is complicated, generally requiring at least two silk screen printed layers, one light blocking layer and one diffusively reflective layer, and may possibly not provide an optimally illuminative effect. Also, most of the above-described prior art signs are single-sided, and signs visible from both sides have to be constructed by placing a pair of signs back-to-back.
There therefore exists a need for an improved internally illuminated sign, which overcomes many of the disadvantages of such prior art signs. Besides the primary aim of improvement in the illumination effect, the sign should preferably have good performance under limited solar illumination levels, should be reliable even in winter time, and should provide an indication of low battery charge capacity and continued operation even when the battery charge capacity is low.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.